[AST] Use invalid conformance for unsatisfied requirement

There is an invariant that SignatureConformances should have the same
size as the number of conformance requirements in the signature.
Previously, since unsatisfied requirements weren't reflected in it,
that caused a crash.

rdar://problem/43625800

lib/AST/ProtocolConformance.cpp

@@ -578,7 +578,8 @@ NormalProtocolConformance::populateSignatureConformances() {
 
       // Allocate the buffer of conformance requirements.
       auto &ctx = self->getProtocol()->getASTContext();
-      buffer = ctx.AllocateUninitialized<ProtocolConformanceRef>(numConformanceRequirements);
+      buffer = ctx.AllocateUninitialized<ProtocolConformanceRef>(
+          numConformanceRequirements);
 
       // Skip over any non-conformance requirements in the requirement
       // signature.
@@ -600,11 +601,20 @@ NormalProtocolConformance::populateSignatureConformances() {
       other.owning = false;
     }
 
+    ~Writer() {
+      if (!owning)
+        return;
+      while (!requirementSignature.empty())
+        (*this)(ProtocolConformanceRef::forInvalid());
+    }
+
     void operator()(ProtocolConformanceRef conformance){
       // Make sure we have the right conformance.
       assert(!requirementSignature.empty() && "Too many conformances?");
-      assert(conformance.getRequirement() ==
-               requirementSignature.front().getSecondType()->castTo<ProtocolType>()->getDecl());
+      assert(conformance.isInvalid() ||
+             conformance.getRequirement() ==
+                 requirementSignature.front().getSecondType()
+                     ->castTo<ProtocolType>()->getDecl());
       assert((!conformance.isConcrete() ||
               !conformance.getConcrete()->getType()->hasArchetype()) &&
              "signature conformances must use interface types");